The present invention relates to a novel and useful method for stabilizing the wavelength of a laser source.
Laser sources are widely used in wavelength division multiplexed systems. In wavelength division multiplexed systems, it is important that the wavelength used is very stable. Although lasers are inherently very stable, increased stabilization of a laser""s wavelength becomes crucial as systems migrate to dense wavelength division multiplexing (DWDM) types. In DWDM systems, many wavelengths are placed on a single fiber to increase system capacity. Currently the spacing in DWDM systems between frequencies is around 100 GHz and can be handled by traditional laser stabilization methods. However, as technology moves toward frequency spacings of 25-50 GHz or less, increased stabilization will be required to prevent interference between wavelengths as the spacings become closer and closer.
Presently, to wavelength stabilize lasers, the wavelength or equivalently the optical frequency of a laser is compared to a stable reference element. One method is to use an optical filter as a reference element. The output of the laser is split and part of the beam is passed through an optical filter to create an optical signal which is a function of wavelength or frequency and optical power (hereinafter xe2x80x9cthe optical filtered pathxe2x80x9d). The optical filtered path is then processed, assuming that a change in signal amplitude corresponds to a change in frequency, and a signal is generated which is fed back to the laser to stabilize the laser""s wavelength. However, a change in signal amplitude at the output of the filter could be the result of a change in the power output of the laser rather than a change in the laser""s frequency.
Another method of stabilizing a laser is to pass a slightly diverging beam of light, obtained by splitting the output of the laser source, through a filter at different angles of inclination as shown in FIG. 1. The two photo-detectors, P1 and P2, act as apertures and capture a different portion of the light emitted by the divergent source. This produces two different spectral responses, offset in wavelength according to their angular difference with respect to the filter. Since P1 captures a portion of the emitted light which passed through the filter at a higher tilt angle than that captured by P2, it""s response will peak at a slightly lower wavelength than P2 as depicted in FIG. 2. The filter and alignment parameter are chosen so that the wavelength offset between the two responses is roughly equal to their effective bandwidths. The signals are then compared differentially to generate a signal which can be used to stabilize the wavelength of the laser by maintaining xcex=xcex0, as further depicted in FIG. 2.
In a stabilized system, wavelength or frequency drift can be introduced by the aging or temperature dependence of the laser itself, or by the aging or temperature dependence of the optical reference filter, the optical detectors, or the stabilization electronics. In addition, manufacturing variations of system components can result in varying wavelengths from system to system. Existing systems are unable to adequately compensate for the multitude of variables that can arise in a stabilization system when a very high level of stabilization is needed.
The present invention provides an improved method for stabilizing the wavelength of a laser source. The invention accomplishes this objective by using an optical filter, dual optical paths, analog and digital conversion, and a microcontroller.
In a preferred embodiment of the present invention, wavelength stabilization of a laser is accomplished via a laser, optical couplers, an optical filter, optical detectors, current-to-voltage converters, amplifiers, analog-to-digital converters, a microcontroller, and a digital-to-analog converter.
In the present invention, a laser generates a signal which is carried by a fiber optic cable. Two separate paths are created from the fiber optic cable via photo-couplers. The first path is an optical filtered path which passes through an optical filter. The second path is a power reference path used for normalization. Since the optical filtered path contains an optical filter, it provides a signal the power of which is a function of wavelength as well as the optical power output of the laser. The power reference path is unfiltered so as to provide a signal the power of which is a function only of the optical power output of the laser. A change in the output power of the optical filtered path should primarily indicate a frequency change of the laser. However, the change may be due to a change in the optical power of the laser. By normalizing the optical filtered path to the power reference path, the change in power in the optical filtered path that is due to frequency change rather than laser output power change can be isolated and used to stabilize the frequency of the laser source.
The other components are used to provide electrical signals, convert the signals into a usable form, and manipulate the signals. Optical detectors are used to convert the optical signals from the optical filtered path and the power reference path to electrical signals. The electrical signals produced by the detectors are then converted from current to voltage, via current-to-voltage converters. The current-to-voltage converters may provide some pre-amplification to the signal or pre-amplification may be provided by other means. Next, the signals from the converters are amplified, via amplifiers, to provide gain and prepare them for analog to digital conversion. The amplified signals are then converted from analog to digital by analog-to-digital converters to prepare them for processing by a microcontroller. The microcontroller then processes the signals in any manner desired using software code and generates an appropriate signal which is converted from digital to analog by a digital-to-analog converter for use in adjusting the laser""s frequency. The microcontroller""s processing can be accomplished by any of the following types of apparatus: microprocessor, processor, digital signal processor, computer, state machine, or essentially any digital processing circuit.
The present invention adds greater flexibility to wavelength stabilization systems. For example, long integration times, which are impractical via traditional stabilization means because of unrealizable component values and physical size restrictions, level shifting or stabilization on either/or both positive and negative slopes, and accommodation of manufacturing variations in the optical filter, are all possible utilizing the present invention.